- Regulatory requirements for floor screeds
- Types of screeds
- The composition and sequence of the device of a cement-sand screed
- The composition of the cement-sand screed
- Preparatory work
- Basic styling process
The purpose of the screed in the main composition of the floor is to level the underlying surface, arrange the required slope for covering or create a hard layer on non-rigid thermal insulation. Our recommendations will help you to correctly perform the screed installation, ensuring the quality and durability of the floor..
Regulatory requirements for floor screeds
The minimum screed thickness is set:
- for the device of a slope on slab floors – 20 mm;
- in slope-forming screeds on a layer of insulation – 40 mm;
- when covering communication pipes at least 45 mm + pipe diameter;
- for dispersion-self-compacting mortar mixtures and concretes not less than one and a half times the diameter of their largest filler.
In the case of large concentrated loads (more than 20 MPA) when laying the screed for heat and sound insulation, its thickness is set using the calculation for compression in place.
When installing screeds based on ready-made dry cement-sand mixtures for flooring, their compressive strength should be:
- for leveling, sloping and concealing communication pipes – more than or equal to 15 MPa;
- for self-leveling floors – more than or equal to 20 MPa;
- on the elastic surface of a heater or sound insulator – more than or equal to 20 MPa;
- for porous compositions more than or equal to 5 MPa.
The class of heavy concrete for monolithic screeds is allowed at least B12.5, and for self-leveling floors and for insulation should be more than B15. Lightweight concrete class with thermal insulation effect – not lower than B5.
When screeds, laid on sound and thermal insulation, are connected to other structures or communications passing through the ceiling, gaps of 30 mm wide are provided in these places. They are made to the full thickness of the layer and filled with soundproofing material. Cannot be cut into cards from in-situ concrete or mortar screeds.
For rooms where temperature drops are possible, a device for the entire thickness of the layer of expansion joints is required, which are filled with an elastic polymer material. They must necessarily be located above the expansion joints in the underlying layer, above the joints in the floor slabs or coincide with the axes of the columns.
The size of the gaps between the two-meter rail and the screed surface should be:
- 4 mm – for gluing insulation, interlayers of cement-sand mortar, polyvinyl acetate-cement-sawdust composition of xylene;
- 2 mm – for linoleum, roll materials on a synthetic fiber base and bulk polymer coatings, as well as coatings made of synthetic resins and glue, which contain cement.
Types of screeds
Structurally, screeds are divided into monolithic (multi-layer and single-layer) and prefabricated. According to the material, they are:
- from light and heavy concrete;
- from asphalt concrete;
- from a cement-sand mixture;
- porous cement screeds;
- reinforced with mesh or fiberglass.
According to the method of laying, screeds are classified:
- with the usual water-cement ratio;
- self-leveling with increased V / C;
- semi-dry with a reduced W / C index;
- dry or prefabricated from ready-made waterproof sheets.
Screeds with a normal W / C ratio are installed on a damp substrate with leveling and compaction to the full depth of the layer. Slope-forming screeds are made only on floors. In floors on the ground, the slope is arranged due to the underlying layer.
In self-leveling screeds, the required level of their surface is set by their own weight. At the same time, there must be good waterproofing under them, preventing the flow of water into the underlying layers..
The semi-dry screed includes fiber, which makes the material more plastic and actually reinforces the layer. When laying it, due to the increased rigidity of the composition, careful alignment and compaction is required to compact the layer. The advantages include a shorter hardening time of concrete or mortar, as well as their increased strength..
A dry or prefabricated screed is made of ready-made large-sized wood fiber, gypsum fiber sheets, particle boards, gypsum boards, and plywood. A device of this type of screed is undesirable in wet rooms. All joints in the prefabricated installation of the screed are glued along the entire length with adhesive tape or thick paper.
By the type of adhesion with the underlying layer of the screed, they are divided into:
- Connected. They are standardized in terms of adhesion strength with the underlying layer.
- Screeds with a release layer. Typically, a waterproofing layer serves as a separation layer. If it is made of a material based on bitumen, and there is a cement binder in the screed, then a mastic based on bitumen with sanding (fr. 1.5–5 mm) is applied to the surface of the insulation.
- Floating. Their use is justified in the case of increased deformability of the underlying floor structure (wooden floor). To prevent damage to the integrity of the screed, it is separated from vertical structures (walls, partitions, etc.) with shock-absorbing gaskets (damper tape).
Screed with a release layer
Installing a damper tape
In order to speed up the process of screeding, you can use ready-made dry special mixtures. Their composition contains mineral binders and modified polymer additives.
Screeds are often used that are made of lightweight concrete or bulk materials impregnated with liquid waterproofing compounds. They create a durable leveling and at the same time heat and sound insulation or waterproofing layer.
Polystyrene concrete screed
The composition and sequence of the device of a cement-sand screed
The composition of the cement-sand screed
For cement-sand screeds with a normal water-cement ratio and a cement grade of at least 400, the following composition by weight is recommended:
- Mortar grade 150:
- cement – 1 part;
- sand – 3 parts;
- water – 0.55 parts.
- Mortar grade 200:
- cement – 1 part;
- sand – 2.8 parts;
- water – 0.45 parts.
Screeds in most floor compositions are performed on the underlying layer, which can be rigid (concrete, reinforced concrete) and non-rigid (asphalt concrete, sand, gravel, crushed stone, slag, etc.). Before starting work on laying the screed layer, it is necessary to prepare its surface.
Dust and dirt must be removed from a solid base, the surfaces of bulk materials must be leveled. For better adhesion to the hard, smooth underlying layer, notches are made on it or treated with metal brushes.
Preparation of the base for the screed
When the screed is laid on a separating waterproofing layer, its surface should be covered with a layer of bitumen-based mastic or tar with sanding. If sand is sprinkled by hand, then its temperature should be 50-60 ° С.
The laying of the cement-sand screed begins after plastering and other works, so as not to further damage the integrity of the layer. At floor level during the screed installation, the air temperature is allowed not lower than +8 ° С.
Basic styling process
The slurry mobility of a standard cone in accordance with the standards is:
- 5–6 mm – for M150 screeds;
- 4-5 mm – for M200 screeds.
The thickness of the screed is determined by the project; at high concentrated loads, it is checked by calculation. Usually it is at least 20–40 mm. The base for the cement-sand screed must be moist, but without the presence of water. The surface prepared for the screed device is marked into strips 2.5–3.5 m wide. Lighthouse strips of wooden bars or rolled metal are laid along the strips along the layer height.
The mortar is laid through the strip along its entire length. The freshly laid mixture must be leveled with a rule and compacted until the moment the solution stops moving or moisture appears on the surface of the screed. For this, vibrating screeds or site vibrators are suitable..
Smoothing and smoothing of the screed surface is carried out before the mixture begins to set. For normal mortar hardening, moist conditions are necessary. After 24 hours, a moist coating is placed on the screed surface. This is done within 7-10 days..
After the strips have completely set, they begin to lay the solution between them. In this case, adjacent areas serve as beacons and formwork. The surface of the screed is prepared for the device on it an interlayer:
- under a primer made of mastics and cement-based adhesives, at the end of the laying, the mixture is smoothed or sanded after it has set;
- milled for polymer seamless coatings.
According to the long-term analysis of the failure of floors, in most cases, the reason for this is poor-quality performance or violation of the screed production technology. Therefore, it is very important to correctly and carefully fulfill all the necessary requirements when installing this layer..